Frequency shift teleprinter



Filed Dec. 19, 1950 5 Sheet s-Sheet l C. A. HIGGINS FREQUENCY SHIFT TELEPRINTER Spt. 29, 1953 5 Sheets-Sheet' 2 Sept. 29, 1953 c. A. HIGGINS FREQUENCY SHIFT TELEPRINTER 5 Sheets-Sheet 5 Filed Dec. 19, 1950 Nun@ X mmm uw; S554 @www Sept. 29, 1953 c. A. HIGGINS FREQUENCY SHIFT TELEPRINTER 5 Sheets-Sheet 4 Filed Dec. 19, 1950 Illll v ON @mme ( JN 22x10 ...IAR o You NM uw Sept. 29, 1953 c. A. HIGGINS FREQUENCY SHIFT TELEFRINTER 5 SheetsQ-Sheet 5 fh Filed Dec. 19, 195o Patented Sept. 29, 1953 UNITED STATES ifPATENT oFFIcE :.-.2,654,c25 I'RllQUENCYT SHIFT TELEPRI-NTER Charle'sAHigginsi-S Boonton, AN J.,r assgno'r! to @Radio1 Frequency.. Laboratories; Inc.,y Boonton,

N. J., avcorpdration of =New yJersey Application December 19,l 1950,*serinNo. ,201,622

A '8"-Claims.

This invention relates to a systemrfor freceiving and translating radio-transmitted'ffsignals and' more particularly to` a 'novel 'electronic l circuit for eiectuating 'the' operation of appropriate apparatus in lresponse to received" radio-*code signals. Y y* y. A

Although vmy jinvention'l has *generar `application to visual indicators, recor'ders;A electronic printers, facsimile receivers; and--thelikefIjshall describe the invention' with\"specic reference to a teleprinter. *Those skilled in this '-art'vvill have no diiiiculty v in 'I applying` the prnciples'fand circuits herein described"V to suclr -othe'rldevices.

In teleprinter vsystems` ofjf'th'el typeto vvlich this invention Amay `be applied; -`vit Vis ycustomary to transmit coded radio 'waves' oftxvofdifferent Y frequencies? in alternate l sequence', jeachi lsuccession of wave pulses-havinga A'precisetime duration measured, generallyg.v in milliesecnds.' `Va, rious charactersgures; punctuationlmarksk etc., are "represented by I individual s combinations? 'of the two Y Wave pulses #inf-accordance With-*the 'standard radiotype r code and`r= these `rlr`coded-"5se- *quences are translated at thereceiver'econv'erter yinto direct current'pulses of corresponding 'time duration. ,Suchdirecti current pulses `-aref"`em ployed toset up l an lelectromagnetic lselector i .mechanism Vpreparatory to imprinting o'f'fg-t'he corresponding fcharacterornumber Aon aeisheet of paper.

While a singleufrequ'ency systemoperat-ing` as a, simple on-Dif codeLarrangementfmayfbe employed for teleprinting. purposes, it f has been Vfound thaty random electrical: andyor radio energy disturbances, either` external? tol`` or i withinthe receiver-convertenf produce* spurious`v operations of the printing mechanism. Consequently; present accepted ypractice employs theftWo-frequency system, said frequenciesrbeing in`v fthe `radio spectrum, say` 2 megacycles'vvith4 a frequency separation of 850 cycles. It is rcommon"1practice` to designate one of the Afrequencies? a's l the iSpace signal and the vother as the Mark/signal. 'ff-Thus, the transmission ofthe letter "Al comprises: a Space signal (2 megacycles) '.for22imilliseconds, followedJ by aMark. signal (.2 m'cill850 cycles) for 44 milli -seconds-f followed f by 'a1' Space? signal (2 mc.) for 66 mini-seconds, followedib'y'aMark signal (2 mc:-{.850 cycles)*1fo`r` 31f=riiill-isecorids -Present teleprinter circuitsf-'empliyf-'aidiscriminator vtype oflid'em'odulatoi'- toldistingiii'shl b ef tween the 4Mark andY Space-signalswhereby only .-the Mark signal will be'effective-tolcauseactuaf tion: of the.selector::magnetfrnechanisln/ n 2 pulsesfwhichvaryin time-duration and time -"-'sp`a'cing; set up the `Aselector@mechanism` for the "'-power operation" of the type'bar corresponding to f" the f particular jicode character received. 1-rv`Hollfevei,""the" discriminator "demodulator f ar- *ran'gexnent issubje'ci'ftov numerousxserious disfadvantages'vvhichmay be summarized as follows:

l; 1 AE i "discriminatorvdemodul'ator "usually re- `quires ltheuse' of 'a D.C=.` amplifier Ifor 'increasing f-thegain "after" the' discriminating!function and it `is "We11'kno`wn 4that"D;C.fampliiers are; in general, difficult toy design and 'unstable in i 'operation. u v "2. A1,discriminatoriicircuit, ofthetypenovir in la* general VVuse, does {not provide equal positive i. and negativevoltagepulse'sffor'operation of the. seu lector mechanism'the' eventthe signalfre- ""quehc'iesj'or Ithe"-re"ceiver oscillator, driftD from -the'mean frequency fvalue." This necessitates recourseto f automatic *frequency `-controls,""' or manual'frequ'ency control underconst'ant supervision by thev'o'pera'tor. A

"3. A*discriminator*circuit is susceptible to spurious loperations by extraneous disturbances and Yis vulnerable tov l adjacent Ychannel. interferencev-unl'ess*very'sharp cut-oi lters areem- *iloyedwv Unfortunately, `sharp cut-oir filters restrict thel amount of frequency drift which can be "tolerated-toa very'narrow range. "Consequently, 'it is necessary toi employa very active automatic "frequency 'controland it is well known that very often"an*interfering signal will take rover. con- *t'rol of-fthe automatic frequency control circuit `causing thejwantedisignal to be lost entirely.

The vpresent-invention is directed to agnovel 'carrierffrequency shift converter circuit Idisp'osedbetween thefradio signal receiverand lthe l`4`relectro-magrietic-- "selector mechanism, which converter operates on 'a'phase shifting and f het- 40 Ierodyning principle to 'overcome-.the objections "to;anddisadvantages in', present teleprinter devices. n

sAno'bject orf-'this invention is the provision of a receivereconverter system inherently capable 'ofhighly reliable operation and one in which the operation'of "thef mechanicalV translating ap- Hparatus is "unaffected by extraneous electrical disturbances, inter-channel interference or-*rela- "tivelywideideviations Iinthe frequency of the received"signals` i Anobject of thisinvention is the provision of a radio "receiverconverterf employing a phase *shifter 'ari'angement"fordistinguishing' Abetween Mark andlSp'ace-'signa-ls, 'whereby extraneous disturbanc'es" erelauwmaticany 4baiancedmt of the circuit controlling operation of the mechanical translating apparatus.

An object of this invention is the provision of a novel electronic circuit for teleprinters, wherein stability of operation is obtained by employing A.-C. amplifiers.

An object of this invention is the provision of an electronic system for operating electromechanical translating apparatus in response to radio signal waves of two different frequencies and comprising a radio receiver, means for translating the received signal waves into two intermediate frequencies, means for demodulating the two intermediate frequency signal waves, means for channeling the demodulated waves into separate channels, means for rendering ineffective the wave pulses in one of the channels, and means for effecting actuation of the electromechanical translating apparatus in accordance with the wave pulses in the other channel.

An object of this invention is the provision of a translating system for operation of a teleprinter, or the like, in accordance with transmitted Mark and Space signals of two different frequencies, said system comprising: a radio receiver converting the transmitted signals to corresponding pulses of intermediate frequency, a first balanced demodulator, a second balanced demodulator, circuit elements impressing the intermediate frequency pulses upon the input of each demodulator, an oscillator tuned to the mean frequency of said intermediate frequency pulses, circuit elements for impressing the oscillator output directly upon the control grids of the first demodulator, whereby the output voltage of the demodulator is an audio frequency pulse, a 90 degree phase shifter connected between the oscillator output and the control grids of the second demodulator, whereby the audio frequency output pulse of the second demodulator is dis- 4 placed 90 degrees from the output of the first demodulator, individual wide-band phase Shifters for splitting the audio frequency voltage output of each demodulator into two components displaced by 90 degrees, means for combining the quadrature voltage output components of both said wide-band phase shifters to produce separate wave pulses corresponding to the received Mark and Space signals, means for converting the separate wave pulses into positive and negative D.C. pulses respectively, and means for effectuating the operation of the teleprinter selector` magnet only by the positive D.C. pulses.

These and other objects and advantages will be apparent from the following detailed description of a practical embodiment of the invention when taken with the accompanying drawings. The drawings are for purposes of illustration and are not to be construed as dening the scope or limits of the invention, reference being had for the latter purpose to the appended claims.

In the drawings wherein like reference characters denote like parts in the several views:

Figure 1 is a block diagram which identifies the various elements of the eelctronc circuit and illustrates the functional relationship between them;

Figure 2 is a vectoral representation illustrating the phase relationship between the voltage outputs of the demodulators and phase Shifters when a Mark and Space signal received;

Figures 3A, B and C comprise the circuit diagram of my frequency shift converter.

The present invention will be described with specic reference to established teleprinter channels, that is, one wherein the transmitted code comprises two frequencies differing by 850 cycles. although my circuit will operate equally well for frequency separations of from to several thousand cycles. It will he assumed the Space signal has a frequency of 2 megacycles (mc.) and the Mark signal 2 mc.-{850 cycles. These frequencies are heterodyned down to an intermediate frequency in the radio receiver and my novel converter is adapted to convert the Mark signal into a direct current pulse which energizes the selector magnet mechanism in the printer, and to balance out the Space signal so that this signal has no effect whatsoever upon the selector mechanism.

If the radio frequency oscillator' of the receiver is tuned to 1,085,425 cycles it will beat with the incoming Mark signal having a frequency of 2,000,850 cycles to produce an intermediate frequency (I. F.) of 915,425 cycles. Likewise, an incoming Space signal having a frequency of 2,000,000 cycles will beat with the oscillator frequency of 1,085,425 cycles, resulting in an I. F. of 914,575 cycles. If the I. F. amplifier' of the receiver has a mean frequency of 915,000 cycles, which is typical of present receivers, it will easily pass frequencies several kilocycles above and below the mean frequency by reason of its wide band-pass characteristic. Thus the Mark signal results in a 915,425 cycle wave and the Space signal results in a 914,575 cycle wave, in the I. F. amplifier. It is here again pointed out that these intermediate frequencies appear in sequence, that is, one at a time, depending upon whether a Mark or Space signal is received.

Referring now to the block diagram, Figure 1, the intermediate frequency outputs of the receiver are amplified by a Cathode Follower Amplifier, a, whose outputs, in turn, are fed, in phase, to the control grids of two Balanced Demodulators #1 and #2, identified by the letters b, b', respectively, in the drawing. A Beat Frequency Oscillator c, is set to oscillate at the mean intermediate frequency, namely, 915,000 cycles and the oscillator output is fed directly to Demodulator #l by means of the two leads carrying voltages that are exactly degrees out of phase with each other. Thus, the incoming frequency, say a Mark signal of 915,425 cycles, beats with the oscillator frequency (915,000 cycles) to produce an audio frequency output of 425 cycles from Demodulator #1. Inasmuch as the demodulator is of the balanced circuit type two 425 cycle waves will appear across its two output leads, said waves 'being exactly 180 degrees out of phase with each other.

The output of the Beat Frequency Oscillator is likewise impressed upon Demodulator #2 but through the 90 Phase Shifter X. Thus, the two leads from the Phase Shifter carry voltages that are 180 degrees out of phase with each other and both of these voltages are 90 degrees out of phase with those fed to Demodulator #1. This produces, in Demodulator #2, a 425 cycle audio frequency output which is, in all respects, 90 degrees out of phase with the 425 cycle audio frequency output of Dernodulator #1.

It will be noted that the Demodulators #l and 2 are connected to Wide Band, 90, Phase Shifters atl and 2, identified by the letters d, cl', respectively. These phase shifter-s comprise rather complex, individual networks each capable of taking a single audio frequency voltage, in the range of approximately 50 to 1000 cycles, and derlving therefrom two voltage components that are very nearly 90 degrees out of phase with each other. The dual,'outputfvoltages-.of theindivid-. ual vPhase nShifters are @combinedinvthe .Adden- Circuit, ye, which, byreasonofthefxed andexact.y phase relationships,,provides an :automatic-,sepas rationuof the Mark-andSpace .signals received by; This function ofthe Phase Shifters f.

the receiver. can best be described by reference `tovector` dla-- grams.

Referring now to the -upper portionofligure 2," there is shown a-qualitative-vector representation' of -the phase relationshipsof-the-various yvoltages= when a Mark signal isapplied-to the network; that is, when the incoming rfrequencyfisvhigher` than that ofthe BeatFrequencyOscillator- As stated herein above, the-Demodulators-are ofthe-l balanced circuit type, whereby two rvoltages api pear across the outputfof each demodu'latorand (input) to 90 (output), asillustrated `in the secf` ond column of Figure 2. Itisapparentthe out? put voltage A1 of the'PhaseyShifter-No. l iseXv.

actly 180 out of phasewiththe `output.,voltage Bz of the Phase Shifter No. 2 and that the output voltages B1 and A2 of the No. l and No. 2 Phasel Shifters, respectively,` are exactly. nphase with each other. These outputvoltages constitute the,

input to the Adder Circuit (as Ishown `in thethird; The, resultant. voltage ,output` of the column). Adder Circuit is showninthe fourth colummit being apparent the voltagQSAi .and Bepancel each tive. Thus, when a Mark signal ,is applied,tothe other and the voltagesBr and A2 aredirectlyaddif network the voltage outputof theAdder Circuit has a magnitude'twicethat of theindividual volt-4 Y ages applied to the Phase Shifters andsuch output voltage has a direction asshown inthe fourth column of Figure 2.

The lower portion vofligureI 2 is a .Simi1a1grep resentation of the phase relationships of the yvarious voltages when a Space ,signal is,appli edto. the network, that is, when theincorning signal has a frequency lower rthan thatof theBeat Fre-- quency Oscillator. In this .casethevoltage outputs B1 and A2 of the No. 1 and No.2 PhasegShiftf, ers, respectively, cancel out andtheputputpfthe Adder Circuit comprises the in-phaseyyoltages yA1,` and B2 having the direction shown. It may here be pointed out that the output voltages ofsthe"L Adder Circuit, representing the individuallyfapplied Mark and Space Isignalrto the network, y

are in quadrature.

Referring again t .Figure 1theinstantaneous voltage output of the Adr- Circuit has,a mag nitude twice that of the .instantaneous input ,volte-f, ages applied ,theretoY by either .ofjthqWide ,Band 90 Phase Shifters andthe voltageoutputporree sponding to a Mark signalis90 out;of, phasev with that correspondllgto ya Space signal. These voltages appear. as separate and-1 distinct; pulses. having a time duration corresponding to. that of f the received code pulses, These quadrature, suc- However, by reason of the*A A1, B1 represent lthe two s cessive voltage pulses are fed, selectively, by the Adder Circuit;1.to ,theiappropriate channel.: Spee'.

cically, the vMark.:` signals` rvoltage is; .fediA f thel .f

Mark Channel Amplier,:f; andthe Space signalsA voltage is -fed to :theSpaceiChannel rnnplifierg.l f. It is here pointed yout.that,.' by,vi1tue ,offthe-.wide

band characteristics.ofLA thelPhase vShifters. N o'. 1

and No. 2, a.90-phase shiftismaintained over a. widev range of audio frequencies,` thereby accom modating appreciable driftl of :the i transmitted frequencies ordrift of the :receiver oscillatoi.

As shown in thefblockdiagram yof Figure 1,;I

provide means, gformonito1ing both channels: singly or simultaneously by head phones,'or. byf

an oscilloscope, forthe purpose of properly tuning:

the radio receiver.

Individual L0wPass\-Filters,1h, h, are inserted:

into the Markand Space channels, respectively;

to ,increase theselectivity of the audio channels, 1f

thereby reducing, or eliminating, adjacent channel radio interference The Low Pass. l3ilt`e'rsy pass audio frequencies from D.-C. 12o-.1,200 cycles.; and attenuation of `more than 40 decibels, with? respect to any frequencies :above1,600.'cycles,.isl

provided.

The outputs of the .individualH Low Pass Filters-f are amplified-by the separate MarkI and Spacey Channel Amplifiers, lc', k,- respectively, -andfcon-U verted tov D.C. pulses=by -the individualy Space`A and Mark Channel Rectiflers, Z,1Z,frespectively.- Such D.C. pulses are--then-fedfto 'the/Output'i Tubes, o, and the circuit-lisso arranged that thee Mark channel pulses produce positive iin-pulseswhich l cause the OutputsTubesto become conducting, thereby energizing'the teleprinterselecv tor magnet. 0n the other hand, `the Space-cham nel pulses produce negative impulses that-cutoffvx the Output Tubes, therebydeenergizing the selector magnet. The specific operation-ofthe' elector magnet in response to the codedimpuls'es; and the subsequent electro-magneticactuationf of the appropriate Aprinting type'fbar are well known and form no part of the present invention; and, therefore, such mechanisms yare neither',

shown nor described.;l Suffice tosay that theselector magnet is in the output-circuit'of the Out-v put Tubes and, therefore; is energized 4imprecise accordance with=the positive impulses impressed upon such Output Tubes, said positive pulses'corlresponding preciselyl in time `duration and spacing to the receivedMark signals.

Figures 3A to 3C, taken together, constitutejaL wiring diagram ofa frequency shiftconverter network made` in `accordance with and vsuitable for a teleprinter.

this invention o In these wiring diagrams the components -identified in theblocki diagram of Figure 1 arecorrespondinglyblocked off, by the double-,row dotted lines, and ,similarly identified.

Referring,nowspecicallyi toK Figure 3A, thein coming signal is taken from, the 3rd I. F.1stage..

of the radio receiver.throughwthe leaclllhLsuch'V signal being heterodyned `down vto ian;intermedii` ate frequency in thereceiver proper, the latterv not shown in the diagram.- As previously stated the oscillator of the receiver is tuned to 1,085,425

cycles and, therefore,A beatsA against an incoming Marksignal, having `Va frequency of -;2,000,'850" cycles, to produce an'intermediate frequency of 915,425 cycles.

Likewise, an incoming 'Space' signal produces a beat frequencyof914,575 cycles.

Consequently, the successiva-incoming Mark and Space signals result in 915,425- and914.57-5V cycle-4 Waves in the lead by the Cathode Follower Circuit, a, injthe tubeA l0. These waves are'ampliied;

7 II, preferably a 6C4 type, which derives its negligible filament and plate currents from the receiver proper. The grid of the 6C4 tube connects to the plate of the 3rd I. F. transformer of the receiver, the other side of which is grounded.

The I. F. signal from the 6C4 tube is applied to the parallel control grids of the SBEG tubes in the Balanced Demodulators No. l and No. 2 through the lead I2 and the coupling condenser I3. Connected to the plates of each pair of 6BE6 tubes are small stepdown transformers I4, I that provide a low source impedance with which to feed the following networks. The swamping resistors I6, I1, I8 and I9, connected across each half of the transformer primaries prevent resonances in the transformers and the variable resistors 20, 2 I, connected across the transformer secondaries may be adjusted to equalize the outputs of the two Demodulators.

The output of the Beat Frequency Oscillator, c, tuned to 915,000 cycles, is fed directly to the paired 6BE6 tubes of Demodulator No. l but it will be noted the oscillator voltages are fed to each such tube in phase opposition. In the case of Demodulator No. 2, however, the oscillator voltages are fed to the paired 6BE6 tubes through the 90 Phase Shifter X. A 6AU6 buffer tube is inserted between the Beat Frequency Oscillator and the phase-shifting network in order to prevent any reaction on the frequency of the oscillator caused by tuning the secondary of the phase-shifting transformer 2|. The desired 90 degree phase shift is obtained by tuning the network connected to the 6AU6 buffer tube and when such circuit is at resonance the be correct.

It will now be apparent that the incoming frequency, say a Mark signal of 915,425 cycles, beats with the frequency of the Beat Frequency Oscillator to produce an audio frequency signal of 425 cycles in the output transformers I4, I5 of the Demodulators No. l and No. 2, respectively. The Demodulators being of the balanced circuit type, two 425 cycle voltages will appear across the secondary windings of the transformers, such voltages being 180 degrees out of phase with each other. These voltage outputs of the individual Demodulators are represented by the vector arrows A1 and B1 (associated with the transformer I4) and A2 and B2 (associated with the transformer I5), it being emphasized that the dual voltages of each Demodulator are 180 degrees out of phase with each other, and 90 degrees out of phase with those of the other Demodulator (see also Figure 2). rlhese del modulator output voltages are fed to the two Wide Band 90 Phase Shifters through the leads E, F, G, and H, which leads tie in the circuits of Figures 3A and 3B.

Referring now to Figure 3B, it will be noted that two, separate Wide Band Phase Shifters d, d', are employed in order to maintain the necessary 90 degree relationship between the output voltages of the Demodulators for, although these output voltages of the Demodulators are 90 degrees apart, the relative phase relationship of these voltages does not necessarily bear a fixed relationship with the input signal voltage. Each Wide Band Phase Shifter serves as a means for deriving, from a single audio-frequency voltage, two new voltages of the same frequency but with the phase angle between the new voltages held constant over a wide frequency range. Each derived voltage has an amplitude charu acteristic linearly variable with the amplitude phase shift will of the input voltage independently of the frequency. Such results are obtained, within each Wide Band Phase Shifter, by two networks whose phase angle increases substantially linearly with the logarithm of the frequency and, when the networks are properly matched, the phase difference between them remains nearly constant over a wide range of frequencies. Although the absolute values of the components in these phaseshift networks are not important, the mathematical relationships between the resistors and the capacitors must be accurately established to maintain the desired degree phase shift. The specic values for these components are given in the wiring diagram to produce a 90 degree phase shift for frequencies between and 500 cycles. However, these components could just as well be given different absolute values to alter the frequency spread from 75 to 450 cycles, without adversely affecting the operation of my device.

The vector arrows A1, B1 and A2, B2, associated with each of the Wide Band Phase Shifters, indicate the phase relationships of the derived voltages constituting the outputs of such phase Shifters. These voltages are impressed upon the control grids of the type 12AU7 tubes in the Adder Circuits No. l and No. 2, identified by the letters e', e. It will be noted that the voltage component A1 of Phase Shifter No. l, and the voltage component B2 of the Phase Shifter No. 2, are each impressed upon the control grids of the tube 22 in the Adder Circuit No. l. Inasmuch as these voltage components are of equal magnitude and exactly degrees out of phase with each other, the total output of the tube 22 is zero. Similarly, the voltage components B1 and A2 of the individual Phase Shifters No. l and No. 2, respectively, are impressed upon the grids of the tube 23 in Adder Circuit No. 2. Since these voltage components are in phase and of equal magnitude they produce, individually, equal and in phase outputs in the tube. Consequently, while the voltage output of the tube 22 is zero, that of the tube 23 is twice that which would obtain as a result of the individual voltage cornponents B1 and A2. Thus, when a Mark signal is received by the radio receiver an amplified voltage pulse appears across the lead M and ground, and no voltage appears across the lead S and ground. Conversely, as will be apparent from the vector representations of Figure 2, a received Space signal results in an amplified voltage pulse appearing across the lead S and ground and no voltage across the lead M and ground. There is thus obtained a complete separation of voltages each corresponding, in a mutually-exclusive sense, to the received Mark or Space signals, each having a precise time duration equal to that of the Mark-Space radio code, and the phase angle between them is 90 degrees.

My frequency shift conversion network permlts the use of conventional audio frequency amplifiers for increasing the gain in the various circuits While the conventional type'of converter employing a cliscriminator type of demodulator usually requires D. C. amplifiers for increasing the gain after the discrimineren It may also h ere be pointed out that the above-described circuit produces constant and equal voltages as long as the signal frequency does not shift beyond a point that would carry both intermediate frequencies (Mark and Space) over on one side of the beat oscillator frequency setting.

The successive, quadrature voltage pulses from 11 from the scope and spirit of the invention as set forth in the following claims.

I claim:

1. A translating system for teleprinters of the type including a selector magnet and operated in accordance with alternately-transmitted radio wave pulses of two different frequencies, said system comprising a radio receiver including means converting the received radio pulses into corresponding pulses of intermediate frequency, a pair of balanced demodulators responsive to the intermediate frequency pulses, means establishing in the output of each balanced demodulator alternate audio frequency pulses corresponding to the received radio wave pulses, means shifting the phase of the audio frequency pulses in the output of one demodulator 90 degrees with respect to the similar output of the other demodulator, means providing two phase quadrature voltage components from each audio frequency pulse in the demodulator outputs, means combining the four quadrature voltage components into a resultant voltage pulse, means segregating the resultant voltage pulses into separate electrical channels on the basis of phase difference, means converting the resultant voltage pulses in each channel into direct current pulses, and control means for operating the selector magnet of the teleprinter, said control means being effective to cause operation of the selector magnet only in response to the direct current pulses in one of said electrical channels.

2. The invention as recited in claim 1, wherein the means shifting the phase of the audio frequency pulses in the output of one demodulator 90 degrees with respect to the similar output of the other demodulator comprises an oscillator tuned to a frequency midway between the frequencies of the intermediate frequency pulses, means impressing the output of the oscillator directly upon one of the balanced demodulators, and means impressing the output of the oscillator upon the other balanced demodulator through a 90 degree phase shifter.

3. The invention as recited in claim 1, wherein the means providing two phase quadrature voltage components from each audio-frequency pulse in the demodulator outputs comprises a pair of wide band 90 degree phase Shifters connected one each to a demodulator output.

4. The invention as recited in claim 1, wherein the means combining the four quadrature voltage components into a resultant voltage pulse comprises a pair of dual vacuum tubes with each of the control grids independently biased by one of the quadrature voltage components.

5. A translating system for teleprinters of the type having a selector magnet operated in accordance with alternately transmitted radio wave pulses of two different frequencies, said system comprising a radio receiver including means heterodyning the received radio wave pulses down to corresponding pulses of intermediate frequency, a pair of balanced demodulators, means impressing the intermediate frequency pulses upon the control grids of both balanced demodulators, an oscillator tuned to a frequency midway between the frequencies of the intermediate frequency pulses, means impressing the oscillator output directly upon the input of one of the balanced demodulators, a degree phase shifter connected between the oscillator output and the input of the other balanced demodulator, a pair of wide band 90 degree phase shifters individually connected to the output of each demodulator, means combining the outputs of both wide band phase shifters to produce resultant voltage pulses corresponding to the alternately received radio wave pulses, means segregating the resultant voltage pulses into separate electrical channels on the basis of phase difference, a first pair of alternating current amplifiers individually amplifying the voltage pulses in each channel, indvidual low pass filters connected in the outputs of such amplifiers, a second pair of amplifiers individually responsive to the outputs of the low pass filters, individual rectiflers rectifying the pulse outputs of said second amplifiers, and control means for energizing the selector magnet of the teleprinter, said control means energizing the selector magnet only in response to direct current pulses of one of the said rectifiers.

6. The invention as recited in claim 5 in combination with a visual indicator and means controlling the energization of said indicator, said last means being responsive to the pulse outputs of the rectifiers.

7. The invention as recited in claim 5 including automatic means energizing the selector magnet when no wave pulses are received by the receiver.

8. The invention as recited in claim 5 including means adapted for selective connection of an oscilloscope into the outputs of the said first pair of alternating current amplifiers, and means adapted for selective connection of ear phones into the outputs of the said low pass filters.

CHARLES A. HIGGINS.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Electronics for December 1945, pages and 151. 

